The evaluation of patients with suspected lumbar radiculopathy is one of the most common reasons patients are referred for electrodiagnostic testing. The utility of this study depends on the expertise of the physician who plans, performs, and completes the study. This article reviews the strengths and weaknesses of electrodiagnosis to make this diagnosis, as well as the clinical reasoning of appropriate study planning. The current use of electrodiagnostic testing to determine prognosis and treatment outcomes is also discussed.
Key points
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It can often be clinically challenging to diagnose lumbar radiculopathy. Electrodiagnostic studies are helpful in this diagnosis because the test is very specific and is therefore a good complement to lumbar magnetic resonance imaging, which is a very sensitive, but not specific, test for lumbar spine disease. In addition, it is the only test that gives information about the physiologic function of the nerve root, or if damage to a nerve root has occurred.
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A thoughtfully planned study can also help rule out competing diagnoses that cause pain or neurologic changes in the lower extremity as well as rule in the diagnosis of radiculopathy.
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The utility of electrodiagnostic studies in the diagnosis of radiculopathy depends on the expertise of the examining physician to plan, perform, and interpret the study appropriately.
Introduction
Lumbosacral radiculopathies were first described by Mixter and Barr in 1934, and electrodiagnosis has been part of the clinical evaluation of this condition for over 50 years. The question of whether a lumbar radiculopathy is present is one of the most common referrals to the electrodiagnostic laboratory. This review describes the value and limitations of electrodiagnostic studies in evaluating for this condition, as well as the technical aspects of planning the optimal electrodiagnostic study to evaluate for the presence of radiculopathy and to rule out competing diagnoses. There is also a discussion regarding the use of electromyography (EMG) to help determine the prognosis and treatment of radiculopathy.
It is not always easy to diagnose lumbar radiculopathy. There are many different medical conditions that cause low back and lower extremity pain, or patients may have more than one disorder. Some patients are vague historians, without a clear recall of their symptoms; sometimes the clinical picture is confounded by issues regarding compensation or blame. The physical examination relies on the patient’s cooperation and may be difficult to interpret. Because of this, it is common for patients to undergo further testing to confirm or rule out this diagnosis. From an evidence-based medicine perspective, it can be difficult to assess the value of these tests, because there is no one gold standard for the diagnosis of lumbar radiculopathy. Therefore, in both research and the clinic, a combination of history, physical examination, imaging, and electrodiagnostic testing is used to come to a diagnosis.
Introduction
Lumbosacral radiculopathies were first described by Mixter and Barr in 1934, and electrodiagnosis has been part of the clinical evaluation of this condition for over 50 years. The question of whether a lumbar radiculopathy is present is one of the most common referrals to the electrodiagnostic laboratory. This review describes the value and limitations of electrodiagnostic studies in evaluating for this condition, as well as the technical aspects of planning the optimal electrodiagnostic study to evaluate for the presence of radiculopathy and to rule out competing diagnoses. There is also a discussion regarding the use of electromyography (EMG) to help determine the prognosis and treatment of radiculopathy.
It is not always easy to diagnose lumbar radiculopathy. There are many different medical conditions that cause low back and lower extremity pain, or patients may have more than one disorder. Some patients are vague historians, without a clear recall of their symptoms; sometimes the clinical picture is confounded by issues regarding compensation or blame. The physical examination relies on the patient’s cooperation and may be difficult to interpret. Because of this, it is common for patients to undergo further testing to confirm or rule out this diagnosis. From an evidence-based medicine perspective, it can be difficult to assess the value of these tests, because there is no one gold standard for the diagnosis of lumbar radiculopathy. Therefore, in both research and the clinic, a combination of history, physical examination, imaging, and electrodiagnostic testing is used to come to a diagnosis.
Magnetic resonance imaging versus electrodiagnostic studies in diagnosing lumbar radiculopathy
Most radiculopathies are caused by root compression, most commonly from intervertebral disk disease or other degenerative changes of the spinal column, such as ligamentous hypertrophy or the bony changes that accompany osteoarthritis. Other compressive lesions can less commonly cause radiculopathy, such as tumors and cysts. Magnetic resonance imaging (MRI) is exquisitely sensitive in detecting these anatomic changes. However, MRI often shows disk disease and other degeneration in asymptomatic people. Lumbar disk protrusions can be seen in as high as 67% of asymptomatic patients older than age 60, and more than 20% have lumbar central stenosis. Therefore, MRI is very sensitive in detecting anatomic changes that could cause a radiculopathy but does not give any information about nerve function or whether these anatomic changes could be a source of symptoms.
There are other causes of radiculopathy besides nerve root compression, and MRI would not be helpful in the diagnosis of these types of radiculopathy. Motor radiculopathy can be seen in patients from varicella zoster virus, even in the absence of skin lesions. Inflammatory mediator cytokines, perhaps from regional disk disease or other factors, can be a source of neuropathic pain and a “chemical radiculitis” without evidence of nerve root compression.
Strengths of electrodiagnostic testing for radiculopathy
Studies have found that needle EMG is very specific in the diagnosis of lumbar radiculopathy when the appropriate electrodiagnostic criteria are used. For that reason clinically EMGs are commonly performed to rule in a radiculopathy, particularly in the following situations:
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To determine if the structural changes seen on MRI are the common finding of an asymptomatic abnormality or are actually causing physiologic abnormalities in the nerve root
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To determine the most likely affected level if clinical symptoms and imaging levels do not match
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To look for physiologic evidence if noncompressive radiculopathies are suspected
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To determine prognosis related to axonal loss
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To search for other causes of neurologic symptoms
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Electrodiagnostic studies for radiculopathy are rarely false positive: if an EMG shows evidence of a radiculopathy, the patient almost certainly has one. When the criteria used for diagnosis are the presence of positive sharp waves and fibrillation in 1-limb muscle plus lumbar paraspinal muscles at the corresponding level, or in 2-limb muscles innervated by the same nerve root, it is 100% specific, both in asymptomatic patients and in those patients with low back pain and sciatica. If evidence of either acute changes or chronic denervation (as demonstrated by more than 30% of motor units are polyphasic, have large amplitude, and have increased duration in a study that uses monopolar needles) is used as the electrodiagnostic criteria, then specificity decreases, but still remains in the range of 81- nearly 100%, depending on the level tested.
Limitations of electrodiagnostic studies because of the nature of radiculopathy
In contrast to the strength of very high specificity, one of the biggest limitations of electrodiagnostic testing for radiculopathy is that sensitivity is not that high. The exact sensitivity cannot be calculated, because of the lack of a gold standard, but it is often noted that a patient may clinically seem to have a radiculopathy that electrodiagnostic testing is unable to diagnose. It is also possible that an electrodiagnostician could determine that a radiculopathy is present, but be unable to ascertain the exact root level involved. Some reasons for this relative insensitivity follow.
Why a patient could have a radiculopathy and still have a normal electrodiagnostic study
- 1.
Inability to detect pure sensory radiculopathies: Clinically, most patients present with either purely sensory complaints (such as pain, parasthesias, or numbness) or primarily sensory complaints with some minimal complaints of weakness. However, because the site of nerve injury is proximal to the dorsal root ganglion in radiculopathies, sensory nerve conduction studies will be normal. Therefore, there is no way for electrodiagnostic studies to evaluate these purely sensory nerve problems. Because there is no gold standard for the diagnosis of radiculopathy, it is unknown what percentage of radiculopathies is purely sensory.
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Subtotal root involvement is the norm in lumbar radiculopathies: This root involvement may include demyelination, which would not cause most of the characteristic changes evaluated for on needle EMG, or limited axonal loss that goes undetected because only a few axons are involved. Therefore even in the presence of a motor radiculopathy, nerve fibers supplying much of the muscle are spared.
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If denervation is balanced with reinnervation, or the denervation is old, no fibrillations will be seen, and the denervation will be missed.
Why a patient could have a radiculopathy, but the level of nerve injury cannot be determined
Imprecision of Myotomal Maps
Many myotomal maps have been published, but the primary root innervation of many muscles remains unclear. Besides a lack of consensus in this area, there is considerable individual variation in the innervation of individual muscles. Because of this, if needle EMG changes are found in a muscle, the examining physician cannot say with 100% certainty what root level innervates that muscle; the examiner can only state what is thought to be the usual level for a typical patient.
Difficulty with Precise Localization of the Lesion in Patients with High Lumbar Radiculopathy
There are 2 issues that make this a difficult condition to diagnose precisely with electrodiagnostic testing. One issue is that lesions of L2, L3, and L4 radiculopathy have very extensive myotomal overlap, so it is often not possible to separate out which of these specific roots is the cause of the electrodiagnostic findings. The second issue is that it is often difficult to separate out plexopathy from radiculopathy at these levels. There are 2 main reasons for this. One is that, unlike brachial plexopathies, there are no good, reliable sensory nerve conduction studies for the nerves that arise for the upper lumbar plexus. If there were, this would give evidence to the electrodiagnostician that the problem is the plexus rather than the nerve root. The second reason is the limitation of paraspinal muscles in separating plexopathy from radiculopathy. In theory, abnormal paraspinal muscles would be expected in radiculopathy and would not be present with plexopathy. However, in reality, sometimes the paraspinal muscles are normal, and a radiculopathy is present; sometimes the paraspinal muscles are abnormal for reasons unrelated to radiculopathy.
Difficulty with Precise Localization of the Lesion in Patients with Lower Lumbar and Sacral Radiculopathies
Because of the anatomy of this region, there are many locations where the nerve root may be injured by a ruptured disk or other compressive force. For example, a disk herniation between the L4 and L5 vertebral bodies (which is the most common level) can affect the L4 root if it is a far lateral herniation, the L5 root if it is a posterior lateral herniation, and the S1–S4 roots if it is a central herniation. Within the cauda equina, roots are packed closely together so it is common for bilateral, multiroot lesions to be found in lumbar central stenosis.
Another study limitation: examiner expertise
Although these studies may seem to be objective; in reality, just as in other diagnostic tests, the skill of the physician performing and interpreting the study is the biggest factor in obtaining accurate results. This issue was studied by Kendall and Werner by comparing the diagnostic impressions of 6 cases of lumbar radiculopathy of an unblinded electromyographer with the impressions of the recorded study by a blinded resident or faculty electromyographer. They found that the overall diagnostic agreement was only 46.9%. Faculty was twice as likely to agree on the final diagnosis as residents, demonstrating that extensive training is necessary to perform these studies accurately. Another study that looked at interrater reliability of needle EMG findings in lumbar radiculopathy used only expert examiners and compared the results of unblinded with blinded electrodiagnosticians. This study found an outstanding overall diagnostic impression agreement of greater than 90% between the unblinded and blinded examiners.
Planning the electrodiagnostic study
As mentioned earlier, the usefulness of electrodiagnostic testing for radiculopathy depends directly on the skills of the physician performing the study, and this applies to study planning as well as study interpretation. In general, there are 2 purposes of the electrodiagnostic study: to determine whether there is electrodiagnostic evidence of a radiculopathy, and to rule in or out competing diagnoses for the patient’s symptoms.
The first part of this section addresses study planning in general. The second part addresses using electrodiagnostic studies to help in sorting out competing clinical diagnoses.
Needle electrode examination
The most important part of the electrodiagnostic testing to diagnose a radiculopathy is needle EMG. Other components may be helpful, but they are used primarily to rule in or out competing diagnoses that could explain all or part of the patient’s symptoms. The presence of positive sharp waves and fibrillations in a myotomal distribution is the most reliable evidence of radiculopathy. Many electrodiagnosticians think there is a window of time when these acute changes are seen: They most likely first appear in the paraspinal muscles by about 7 days, but may not be seen in distal muscles for 5 or 6 weeks. Total myotomal involvement is rare—often many muscles within a myotome never show damage. If no further nerve damage occurs, these spontaneous changes generally disappear by about 9 months. This limited duration of findings has also been shown in animal studies. Other spontaneous activity, such as fasciculation potentials and complex repetitive discharges, are sometimes present and may help make the diagnosis. Abnormal motor unit action potential recruitment in a neurogenic pattern may be seen. Chronic neurogenic motor unit action potential changes are frequent in chronic radiculopathies, but if this alone is used for making the diagnosis, there is a significantly higher incidence of false positives, and many electrodiagnostic physicians think that this makes it unacceptable sole diagnostic criteria ( Fig. 1 ).
Related posts:
The Electrodiagnosis of Neuromuscular Disorders
Electrodiagnosis of Cervical Radiculopathy
Electrodiagnosis of Brachial Plexopathies and Proximal Upper Extremity Neuropathies
Electrodiagnostic Testing in Lumbosacral Plexopathies
Electrodiagnosis of Myotonic Disorders
Electrodiagnostic Evaluation of Myopathies
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